No shore diving in Key Largo if that's a consideration.
What about a liveaboard? Takes care of the single supplement issue if you don't mind sharing a cabin. Or if they're not full, you may get your own anyway. More dives and the room/food is covered. They're all Sat->Sat. departures.
You might see if there's anything available on the Aggressor liveaboards. From your area, probably the Cayman or Belize boats could be options since you could get there early enough Sat. afternoon for the sailing. They're offering $500 off on all cruises starting today.
Aggressor Fleet and Dancer Fleet - Luxury LiveAboard Diving - click reservations and you can find what's available.
The Cayman boat docks in Georgetown - 2 miles from the airport. I believe both Belize boats pick you up at the airport.
Blackbeards could be an option for a lot less money ($979+tx/fees) also. All you'd have to do is get to Nassau if they have space. And take a cab to the port.
Bahamas Scuba Diving Liveaboards - Blackbeard's Cruises This close to departure I'd call - 800.327.9600 Space is pretty tight so don't bring anything extra. Also everything is shared, showers, the heads, etc. Most of the bunks are 4 to a room also. It's pretty much like camping at sea.
Also check Cozumel as I believe there's shorter flights there from the Midwest. My friends stayed at Villablanca Garden about 2 mi. from town. They work with three local dive shops and there's an average shore dive off the property. Nice thing is their single/double occ. rates are the same. Not a lot nearby otherwise though.
Dive Packages | Villablanca | Mexico
Or Scuba Club Cozumel but that's more expensive IIRC. There's also affordable options in town, I just don't know any.
I just checked Indianapolis departures for all these destinations and they're all doable. Pricier flights but not too bad considering how close it is. I use this for research, you'll have to book with the airline/travel portal though.
Matrix - ITA Software by Google
good luck,